The present invention relates to a light emitting diode print head for electrophotography, and more particularly to a dynamic drive light emitting diode print head comprising an upper and a lower substrates that are joined together.
Generally, the light emitting diode print head is employed in a non-impact printing mechanism, and comprises a plurality of chips each having a plurality of light emitting diodes arrayed. Parts of a charged body are discharged so that the light beams produced by the light emitting diodes of the chips may be focused on the surface of the drum through the self-focus lens array. The drum is rotated so as to pass the discharged parts through a developing means, transferring means and fixing means, thus printing images on paper.
There are generally two methods for driving the light emitting diode array chips, i.e. the static drive method and dynamic or matrix drive method. In the static drive method, a plurality of drive integrated circuits (hereinafter referred to as drive IC) are mounted on both or either side of the light emitting diode array chips so as to cause the bits of the drive ICs to respectively drive the light emitting diodes. In the dynamic drive method, there are simultaneously driven a plurality of light emitting diodes connected in parallel to each of the bits of one or more drive ICs, and the wiring between the drive ICs and the light emitting diodes constitutes a matrix, thus the method often being called the matrix drive method.
Referring to FIG. 1, illustrating a conventional light emitting diode print head for the dynamic drive method, a lower glass substrate 6 has common electrode 2 and individual electrode wiring 4. On the lower glass substrate 6 is mounted an insulating layer 8, on which in turn are mounted a plurality of light emitting diode array chips 10. A self-focus lens array 12 is mounted over the light emitting diode array chips 10. The under surface of the lower glass substrate 6 contacts a heat sink 11. A case 14 encloses the heat sink 11, lower glass substrate 6 and insulating layer 8 mounting the light emitting diode array chips 10 except the self-focus lens array 12.
Meanwhile, a drive IC 16 for driving the light emitting diode chips 10 is mounted on both or either end portion of the lower substrate 6 with the bits connected to the individual electrode wiring 4. The light emitting diode array chips 10 are connected to the common electrode 2 by means of epoxy resin or direct contact. The width of the insulating layer 8 amounts to about a half of the width of the lower substrate 6, and each of the electrode pads of the light emitting diode array chips 10 is electrically connected to the individual electrode wiring 4 by means of wire bonding. The common electrode 2 and the signal part of the drive IC 16 are connected to a flexible printed circuit, and then to a external drive circuit constructed separately from the light emitting diode print head, thus being connected to the system via the connector of the external drive circuit.
Referring to FIG. 2 for illustrating an enlarged view of the portion A of FIG. 1, the conventional light emitting diode print head more specifically comprises the lower glass substrate 6 mounting the common electrode 2 and individual electrode wiring 4, the insulating layer 8 laid on the upper surface of the lower substrate 6, a plurality of the light emitting diode array chips 10 mounted on the insulating layer 8, and wire bonding 20 for connecting the electrode pads 18 and the individual electrode wiring 4.
Referring to FIG. 3 for illustrating a cross section of the light emitting diode print head taken along line a--a' of FIG. 2, there are shown the lower substrate 6 having the common electrode and individual electrode wiring, insulating layer 8 laid on the upper surface of the lower substrate 6, the width of the insulating layer amounting to about a half of the width of lower substrate, and a plurality of light emitting diode array chips 10 mounted on the insulating layer 8 and connected to the individual electrode wiring (not shown in FIG. 3) via the wire bonding 20.
There is shown an equivalent circuit of the conventional light emitting diode print head. The bits of the drive IC are connected to the individual electrode wiring 4. Dynamic wiring section 22 comprises a plurality of light emitting diode array chips 10 each consisting of a plurality of light emitting diodes connected to the individual electrode wiring 4. The external drive circuit section 30 comprises a control circuit 24 and a plurality of transistors 26 each connected to corresponding light emitting diode array chip 10. The control signal, print data and selection signal of the system are connected to the wiring section 22 via the control circuit 24. Each transistor 26 of the external drive circuit section 30 has the base connected to the selection data of the control circuit 24 with the collector and emitter respectively connected to the common electrode 2 of the light emitting diode array chip and ground voltage.
In operation, if the print data and selection data are respectively applied through the control circuit 24 to the input of the drive IC 16 and the bases of the transistors, light is emitted only from the light emitting diode of the light emitting diode array chip connected to the transistor selected by the selection data. Of course, the light emitting diode that emits the light are those selected simultaneously by the print data. For example, the selection data SDn--1 corresponding to the "n--1"print data PDn--1 is applied to the "n--1"th transistor, light is emitted only from the light emitting diode corresponding to the "n--1"th print data PDn--1 of the "n--1"th light emitting diode array chip.
In such a dynamic drive method, if only one drive IC is used, n times sequences are required to print a line. If two drive ICs are used, ##EQU1## times sequences are required to print a line because of turning on two transistors and two light emitting diode array chips.
The light emitting diode print head for such a conventional dynamic drive method needs a number of connector parts because the head substrate section and the external drive circuit section are separated from each other. Furthermore, the electrical connection between the common electrode and the light emitting array chips is achieved by using epoxy resin or direct contact, thereby increasing the possibility of a short circuit.
In addition, insulating layer should be employed to insulate the lower substrate and the light emitting diode array chips, thus increasing the processing steps of fabrication. Further, when mounting the light emitting diode array chips on the insulating layer, it is essential to make a precise alignment between the electrode pads of the chips and the individual electrode wiring.
Moreover, because of simultaneously forming the common electrode and individual electrode wiring in the lower substrate, the remaining area of the substrate except the area occupied by the common electrode is too limited to contain the expanded individual electrode wiring when fabricating a light emitting diode print head of high resolution, so that it is impossible to achieve the straight wire bonding.
The connection area between the common electrode and the light emitting diode array chips is also too limited to increase the current flow in order to enhance the light output. Besides, it is very limited to increase the printing speed because the drive IC should be mounted on either end portion of the lower substrate.